UbuntuOne seems to have another depressing problem that will cause it to fail compared to DropBox or SpiderOak or similar solutions.
UbuntuOne will only run on Ubuntu (and possibly Debian with a bit of .deb hackery). If you're forced to use Windows at work or CentOS or something, you're SOL.
With DropBox, it works fine on every recent version of Windows, every recent version of Mac OS X and at least currently, any linux distro with Nautilus installed (though i hear there's a command-line DropBox client on its way that has very few dependencies. Likely only simple stuff like rsync or curl or wget or similar).